This invention relates to methods and apparatus for generation of electricity from carbon containing fuels. More specifically, the present invention relates to hydrogen production and related energy production by gasification of coal.
International concerns over global warming are increasingly focused on the role of atmospheric greenhouse gases such as carbon dioxide. The increasing atmospheric concentration of carbon dioxide and its role as a greenhouse gas are resulting in various national and international efforts to either reduce the overall emissions of carbon dioxide or sequester such emissions for isolation and disposal into carbon dioxide disposal sinks other than the atmosphere.
Power plants fueled with gaseous, liquid or solid carbonaceous materials are a significant source of carbon dioxide, yet generation of electricity using such power plants remains a necessary source of electrical power in many countries for the foreseeable future. Unlike automobiles and other dispersed sources of carbon dioxide, power plants burning carbon containing fuels constitute a relatively limited number of large stationary point sources of carbon dioxide. As such they are likely targets of various regulatory initiatives being considered, including, for example, energy taxes, emissions taxes, and mandated carbon dioxide scrubbing measures. Consequently, there is a need for improved methods of extracting the energy of combustion from carbon containing fuels such as coal while also enabling sequestration of the resulting carbon dioxide for disposal by various means.
Considerable effort has been expended on developing various methods for the gasification of coal for purposes such as the reduction of air pollution or production of a more transportable, gaseous source of energy. The well known water-gas production reaction is one method of producing hydrogen from coal:
C+H2Oxe2x86x92CO+H2xe2x80x83xe2x80x83(1)
A related reaction is the water-gas shift reaction:
CO+H2Oxe2x86x92CO2+H2xe2x80x83xe2x80x83(2)
The water gas production reaction (1) can be combined with the water gas shift reaction (2) to produce additional hydrogen. The net reaction is:
xe2x80x83C+2H2Oxe2x86x92CO2+2H2xe2x80x83xe2x80x83(3)
The net reaction (3) is highly endothermic at standard conditions, i.e., at approximately ambient temperature and one atmosphere of pressure, on the order of 170 to 180 kJ/mole, when liquid water is used. (References to endothermic, exothermic and heats of reaction herein refer to standard conditions, i.e., approximately ambient temperature and approximately one atmosphere of pressure.) All three of the above reactions produce hydrogen, and each requires separation of the resulting hydrogen from the other gaseous reaction products for practical application in which carbon dioxide-free emissions are desired.
It is also well known that direct hydrogenation of coal, using gaseous hydrogen at elevated temperatures, can be achieved to produce gaseous reaction products consisting primarily of methane, by the reaction:
C+2H2xe2x86x92CH4xe2x80x83xe2x80x83(4)
The advantage of this reaction is that it is exothermic (75 kJ/mole). However, it requires a source of hydrogen.
It has also been known to use a calcium oxide based process for generating hydrogen from carbon. The summary reaction for this process is:
CaO+C+2H2O(1)xe2x86x92CaCO3+2H2xe2x80x83xe2x80x83(5)
The advantage of this reaction is that it is essentially energy neutral, being exothermic to the extent of only about 0.6 kJ/mole when using liquid water.
This reaction (5) has been utilized in a single reaction vessel in the process disclosed in the paper entitled xe2x80x9cCO2 Acceptor Process Pilot Plantxe2x80x941976,xe2x80x9d published in the proceedings of the Eighth Synthetic Pipeline Gas Symposium, American Gas Association, Oct. 18, 1976. However, there have been difficulties in conducting this reaction, especially to high fractional completion, because the mixing of coal and calcium oxide poses several problems. The coal produces ash, which reacts with the CaO to produce various silicates which interfere with the reaction. Other impurities in coal, such as sulfur, also interfere with the carbonation of CaO to CaCO3.
Accordingly, it is an object of the present invention to provide an energy efficient method and apparatus for the production of hydrogen from coal, other fossil fuels, or other carbonaceous substances.
It is also an object of the present invention to produce hydrogen from coal while also producing carbon dioxide in a substantially pure stream such that it can be sequestered and disposed of to a sink other than the atmosphere.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. The claims are intended to cover all changes and modifications within the spirit and scope thereof.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, there is provided a method and apparatus for calcium oxide assisted hydrogen production from carbonaceous materials such as coal. The two-step invention process includes two separate groups of reactions conducted in separate vessels. Gasification of coal by hydrogenation in a gasification vessel is followed by hydrogen production from methane and water that is driven using a calcium oxide carbonation reaction in a carbonation vessel. In the gasification step, coal is hydrogenated with hydrogen to produce a gaseous reaction product consisting primarily of methane. This gaseous reaction product is conveyed to the carbonation vessel, where it is reacted in a carbonation reaction with water and calcium oxide to produce hydrogen and solid calcium carbonate and to remove carbon dioxide from the product gas stream.
A portion of the hydrogen produced in the carbonation reaction may be returned to the gasification vessel to provide the supply of hydrogen for the hydrogenation of the coal.
In accordance with one aspect of the invention, another portion of the hydrogen produced in the carbonation vessel may be used to heat a calcination vessel, where the calcium carbonate produced in the carbonation reaction is calcined so as to produce regenerated calcium oxide, which is in turn returned to the carbonation vessel. Calcium oxide may be continuously introduced into the carbonation vessel where it reacts with the carbon dioxide produced from the reaction of the methane with the water to form calcium carbonate, which may be withdrawn from the carbonation vessel to be calcined and recycled back into the carbonation vessel as calcium oxide, in a multi-pass loop process.
The amount of hydrogen produced in the carbonation vessel is sufficient to both regenerate the calcium oxide necessary for the hydrogen production in the carbonation vessel and to hydrogenate the coal in the gasification vessel, and yet still provide a net output of hydrogen for the production of electrical energy or for other purposes. Importantly, substantially all of the carbon initially introduced as coal into the gasification reaction ultimately emerges from the invention process in a stream of substantially pure carbon dioxide from the calcination reaction.
In accordance with another aspect of the invention, the water needed for the hydrogen production in the carbonation vessel may be introduced into both the gasification vessel and the carbonation vessel, and allocated between the two reaction vessels so as to separately minimize the net heat generated in each reaction vessel. That is, water introduced into the gasification vessel is vaporized and absorbs part of the exothermic heat of the gasification reaction, and in addition reacts to some extent with the coal to produce carbon monoxide and hydrogen by the water-gas production reaction which is an endothermic reaction.
The sum of the reactions in the carbonation vessel can be either endothermic or exothermic, depending upon whether water is introduced as liquid or steam and depending upon the particular mix of gases coming into the carbonation vessel from the gasification vessel. By regulating the flow of liquid water into each reaction vessel, the net reactions in each reaction vessel can be maintained in an approximately energy neutral state, thus avoiding the need for any other heat transfer mechanism.
In accordance with yet another aspect of the invention, the produced hydrogen may be used to produce electricity in a turbine, high temperature fuel cell or other high temperature electricity generating device. Heat from the turbine, fuel cell or other high temperature electricity generating device may be used-to calcine the calcium carbonate produced in the carbonation vessel.